The correct answer is purchasing of Alaska.
William Seward was the Secretary of State under president Andrew Johnson. As Secretary of State, Seward was interested in purchasing Alaska from Russia during the middle of the 1860's. In 1867, he was able to get a bill passed by Congress that allowed him to buy Alaska from Russia.
Many Congressmen who voted against this purchase dubbed it "Seward's Folly." This is due to the fact that they viewed this purchase as a bad idea, as they felt that Alaska would add no significant value to the United States. However, gold was later discovered in Alaska which helped support Seward's decision to buy this land.